The present invention relates generally to time domain reflectometry measurements and more particularly to a method of reducing time drift of a step stimulus originating from a sampling system, such as an oscilloscope.
Time domain reflectometry (TDR) and time domain transmission (TDT) measurements on a device under test (DUT) are performed with an oscilloscope by applying a step stimulus to the DUT and acquiring the transmitted step stimulus and the reflected signal from the DUT. The step stimulus provides an incident pulse with the reflected pulse depending on the impedance in the DUT. For a rising step stimulus, an ideal reflected response would be a matching rising step stimulus delayed in time for a DUT having an open termination. An ideal matching falling step stimulus delayed in time represents a DUT having a shorted termination. A response having no change in amplitude over time represents a DUT having an impedance mating the step source impedance. In actual use, the reflected response may include more than one change in the impedance of the DUT as represented by various signal level changes in the reflected signal.
Electrical drift occurs within an oscilloscope due to environmental conditions, such as temperature and airflow. Drift can also occur due to operational characteristics of analog and digital components used in the system. When drift occurs, the step stimulus displayed on an oscilloscope will shift position to the right or left on the display relative to the time reference point. The shift can be thought of as a change in the timing of the system (i.e oscilloscope). Under normal usage, the waveform can be “moved” in a similar fashion by changing the horizontal position controls on the oscilloscope or the step positioning control. However, drift is when the waveform move without changes in the control parameters.
When a step signal is used for the characterization of a DUT, time stability is critical for several reasons. Any drift in the TDR step stimulus will limit the ability to enhance the accuracy and resolution of the TDR/TDT measurements by calibration and post-processing of the acquired data. In order to improve the accuracy and resolution of a TDR measurement, the effects of the interconnect to the DUT needs to be removed through a calibration process. Some of the aberrations due to the interconnect can be removed by applying the step stimulus to a standard reference impedance (typically 50 ohms). This reference waveform carries the signature of the interconnect, and can be subtracted from the data reflected from the DUT. If a time drift shifts the position of the step, the interconnect aberrations on the two waveforms will not match, thus the subtraction operation will amplify the effects of the aberrations rather than reduce them. When a mismatched difference waveform is applied for further processing, the resulting waveform will show excess ringing, thus reducing the accuracy of the measurement.
The accuracy of rise time measurements could be affected by the time drift of the step. The TDR resolution is directly proportional with the rise time of the step, thus rise time measurements are fundamental for TDR/TDT characterization. The need to reduce noise and effects of jitter impose the requirement to heavily average the data acquired from the DUT. Depending on the average count and rate of drift, the rise time measurements will loose accuracy when the step edge drifts.
What is needed is a method to minimize or significantly reduce drift of a step stimulus from an oscilloscope for more accurate TDR/TDT measurements.